For a number of years magnetography has been used with increasing frequency and satisfactory success in non-destructive material testing. Recently, it has also been used for detecting defects in welding seams of complex shapes on pipe constructions on underwater drilling platforms. In this case, tests to be carried out below sea level are made by divers.
Among other advantages offered by the magnetography, its increasing popularity is due primarily to the high sensitivity achieved by its use, which sensitivity results from the fact that the magnetography offers far better possibilities than all the other scanning magnetic stray flux testing methods to scan the stray flux in the immediate neighborhood of the test piece surface.
As compared to the well-known magnetic-powder testing method, which likewise offers high sensitivity, magnetography has the advantage that indications as to the depth of the defect can be derived from the defect signal amplitude. However, both the sensitivity and the correlation between defect signal amplitude and defect depth are confined to certain limits. While in the case of level or uniformly curved surfaces, the flexible storage tape adapts itself ideally to the shape of the surface, this is not possible to the same degree in the case of nonuniformly curved or irregular surfaces. Here, the testing quality depends on whether or not and to what degree the storage tape can be pressed into the irregularities of the surface during the magnetization process. According to one known technique, this can be done by means of contact rollers which may be provided with a specific surface profile. Apart from the fact that frequently the use of such contact rollers is impossible, for instance in cases where magnetic pulses are used for the magnetizing process, they also often fail to achieve a sufficiently intimate contact between the storage tape and the test piece surface, in particular when irregularities with small radii of curvature are encountered on the surface. In addition, handling of the contact rollers is complicated, which makes them often undesirable. The same applies also to the use of brushes as a contact medium.
In accordance with another known process, the strip-shaped conductor used for magnetizing the test piece includes an expanded rubber lining serving to press the flexible storage tape against the test piece surface when the conductor is applied to the latter to effect the magnetization. But, of course, this method also does not enable the storage tape to be pressed into sharp-edged irregularities in the surface.